F25, 35i, N55 small scale rough idle

[Adak]

Hi All!

After selling E83 35d, I bought F25 35i, N55, 2011.

Quite a lot milage (143k km), but still in a very good shape.

I've detected one issue and I would want to fix it:
It doesn't matter in which gear the car is, although it is best noticable in D and R, but from time to time engine cannot keep constant revs in idle. Usually its fine, but it is also visible on the diagnostic computer that when revs are ~680 they can also get down to 630 or reach 690. Where 690 is not a problem, at 630 driver feels entire chassis shaking and moving a bit. It lasts for half a second, next few seconds are fine, and it's doing it again. It's quite irregular, but totally repeatable. It doesn't matter if engine is cold or hot but it is visible when idle drops to ~680rpm.

What is already checked:
- spark plugs are fine
- coils are fine
- injectors are fine
- changed 2nd lamba (this behind a cat) as it was suggested by a person with super similar problem - no effect
- new battery

What's next to be checked? There are no errors visible on a computer, car normally accelerates (it's quite dynamic), has enough power, fuel consumption is normal, does not smoke from the exhaust. It does not ever shakes if revs are higher than idle, entire problem is also not visible on the rev meter (saw many posts with "rought idle" and "pulsing revs", but in my case those irregularities are super small, and needle doesn't even move). You can just feel it on the seat, on the wheel etc. Carbon deposit in intake manifold? Throttle? High pressure fuel pump?

I've still got to check vanos (seals were replaced recently), as there was a service recall for it, but I don't think one would be resposible for it.

[SlackTube]

Fuel quality and alcohol content can cause similar issues, coil starting to fail or injector issue also possible.
If there are no codes I'd start by switching gas stations for a few tanks and see if it clears up

[Adak]

Quite sure it's not the fuel, as I used numerous different gas stations, but for sure I'll check coils and injectors again.

I should know (or I hope I will) something more, as I'm scheduled for a diagnose on Wednesday.

[my1stE30]

small engine leaks can cause this response that you describe (personal experience) .... whenever you have a small oil leak that is also a source of a small air leak that will affect the engine at low idle speeds, and eventually can affect the starting of the engine...

I suggest looking for small engine leaks, around the oil filter housing gasket, any vent hoses on the side of the engine, and possibly the valve cover gasket

[najnad]

one or more vacuum lines has a small crack in it...

[Adak]

Will check it, however I'm not super convinced:
Wouldn't a crack in vacuum line auto-seal after engine and engine bay gets hot? Had exact this kind of behaviour in 35d (pulse idling when cold, ideal when hot).

[najnad]

crack in vacuum lines might not auto-seal once they get hot...depends on the crack

[BMW323iDude]

If it gets too troublesome to find you may have to run a smoke test. Valve cover gaskets, cracked plastic emission components, cracked vacuum lines are always suspect in BMWs.

[Adak]

Unfortunately Vanos check proved nothing (bolts were replaced), but valve cover gasket was also replaced - also no joy.

I should know something more next week, as this time I'm going to focus on those vacuum lines.

[Adak]

Vacuums lines are fine. Still rough idle.

Eh... What to do next?

[BMW323iDude]

Were you able to check for carbon buildup? May need to look into getting a walnut blast to clear up the carbon. My N55 is experiencing similar issues and that will be the first thing I do when I get a chance. #directinjectionprobs

[Adak]

I scheduled walnut blast for... Septermber (no earlier dates available). Never really checked how much carbon is there but after 143k km there may a bit.
Question is: can carbon deposit in intake manifold cause rough idle (as described) and absolutely no other issues?

[Adak]

I also replaced spark plugs but no joy - it still idles poorly and drives great.

Planning to swap fuel injectors with test ones and see what happens. Unfortunately I need to wait for that few weeks until test ones are available at my garage.

[BMW323iDude]

I would say wait on changing injectors until after your walnut blast. If your problem isn't vacuum related then I have a strong suspicion it's the carbon buildup. Lots of info on multiple BMW forums related to idle/performance issues with the N55 stemming from carbon buildup.

[Adak]

You are right, that's why this will be only a test on used ones, without actual buying the injectors (yet) if it proves nothing.

I would say both elements (carbon deposit and worn injectors) can cause very similar effects, as I found those claims:
- "replacing injectors caused that my car drives and idles like new!"
- "performing walnut blast caused that my car drives and idles like new!"

Knowing my luck, whatever I will try first won't help, but screw it - I want to fix it.

[najnad]

Did you find a resolution to your problem?

[Adak]

Did you find a resolution to your problem?

Sadly - no.

I did walnut blast few weeks ago (have pics from before and after) and it changed nothing. I will proceed with replacing the injectors with test ones - should happen within few weeks too.

I would say those really may be injectors - it idles a bit better when warm, especially after longer ride.

[Adak]

It's definately not better, I would say it's even a bit worse than the last time.

So after replacing water pump (and also get towed for 350+km cause of it's failure...) I'm replacing fuel injectrors (tomorrow) with different set (used one). I would've bought new but... noone gives me the guarantee that injectors are causing this rough idle.

Will see if there is any difference in idling and how the old ones look.

[Adak]

Aaaand... Nada. Nothing. Zero.

Injectors replaced: 0 improvement.

To be honest: I don't have more ideas. Super-secret crack (that was previously omitted) in a vacuum line? High pressure fuel pump? Nah, if that's a pump it would have caused issues on high revs. Maybe vanos solenoid? But absolutely no such errors :/

Guys, please start up your crystal balls and help me here :-D

[Adak]

Just 2 short clips ilustrating the issue.

This shows how it is heard on the exhaust side (excuse me: it is farting from time to time):


This shows what happens with belt tensioner (replaced 1 week ago!) when it rough idles; it is also very well heard here what exactly is felt inside the car:

It's rather the effect than the cause.

[my1stE30]

Vacuums lines are fine. Still rough idle.

Eh... What to do next?

you state that vacuum lines are fine... how was this determined? There are other locations possible for small leaks that will affect idle. A smoke test with a proper smoke machine run using proper specs is the only way to find errant and enigmatic leaks.... have you had a smoke test performed?

[Adak]

To be completely honest, no actual smoke test was done (closest dealer seemed to be quite surprised by this suggestion). I know that guys had some kind of spray (with isopropyl alcohol?) and they told me that entire engine bay was sprayed with no changes in idling. Is that a proper test?

Will also try to find someone with smoke machine (well, YT shows it as kinda easy task) and some experience to check this near my vicinity.

[BimmerDave007]

Any updates to this or a resolution? I’m having similar trouble with mine. Mine seems worse than the vids tho.

[BMW323iDude]

Hate to hear you've gone through all these remedies without a remedy to your issue. Doesn't sound too bad from the videos you've posted but I can hear some of the slight erratic-ness in idle.
I tend to notice my issues more when it's very hot outside.
I recently changed my plugs and came across some very brittle hosing and emission lines. Always a good idea, as suggested to look at those eventhough it can be a bit tedious.
I did also have issues with our N52 X3. It had the classic faulty CCV valve which required complete replacement of the valve cover and gasket. That resolved our issues with the N52 but I'm not sure if the CCV issues translate to the N55.

[BimmerDave007]

Mine almost dies. I’m considering getting it walnut blasted. Perhaps it has carbon build up.